Hi,
same issue when I'm at home (wi-fi) and away (LTE).
To be more specific the issue only persists on home screen. It rings full 6 times when the phone is locked and when the app is opened of course.
To be even more specific after 2 rings on home screen, the Zoiper banner disappears and it stops ringing but when I open app it's still ringing and I can answer the call.
Best regards
David.